The use of lightweight non-lethal projectiles in place of conventional bullets has recently increased. Projectiles made of lightweight polymers are used by the military and law enforcement in conventional or specialized training firearms for training and non-lethal applications. Similarly, spherical polymer projectiles are also often used in air guns and other pneumatic guns for recreational use. These projectiles often comprise low-weight and/or frangible materials such as lightweight polymers that transfer less energy to the target than conventional bullets causing significantly less or no damage to the target. The lower weight of the polymer materials as compared to heavier conventional bullets or metal pellets allows the projectiles to be fired with a reduced propellant charge or by a lower pressure compressed air and travel at a lower speed to further reduce the likelihood of damage to the target.
The drawback of using conventional lightweight polymer projectiles is that the lower relative weight used to reduce the momentum of the projectile and consequently the damage caused by the projectile impact also inherently worsens the ballistic characteristics of the projectile. Specifically, the lower weight reduces the effective range in which the projectile can be fired with reasonable accuracy. A common approach to improving the effective range of the lightweight projectile is to increase the muzzle velocity of the projectile by increasing the propellant charge or providing additional compressed air or gas. This increased muzzle velocity compensates for the reduced weight to increase the effective range of the projectile increases. However, the increased muzzle velocity creates a standoff distance within which the projectile is travelling sufficiently fast to possibly cause excessive or lethal damage to any impacted target. As a result, compensating for the reduced weight of the projectile by increasing the muzzle velocity of the projectile also increases the standoff distance.
The rifled barrel found in many conventional firearms is also used to improve the effective range of conventional bullets by imparting a spin to the bullet as it travels through the barrel. The spin stabilized bullet has a greater effective range at which the bullet can be fired accurately than a bullet that is simply fired through an unrifled barrel. Although the contact between the bullet and the rifling inside the barrel etches striations into the exterior of the bullet, the bullet will travel through the barrel with minimal friction after the initial engraving of the metal exterior or jacket. Moreover, the heavier weight and faster velocity of conventional bullets minimizes the effect of the friction caused by the contact between the bullet and the rifling.
In contrast, the significantly reduced energy of the lightweight projectile caused by the reduced weight and velocity of the projectile maximizes the effect of the friction between the projectile and rifling. In addition, the friction caused by the contact between the metal rifling and lightweight polymer projectiles is often significantly greater than the metal-metal contact between the rifling and a conventional bullet. Moreover, the spherical pellets used in most air or pneumatic guns do not have the requisite surface area contacting the rifling to receive the necessary spin from the rifling. However, the spherical pellets are favored in air guns as easier to load and handle within the air gun.
Another drawback is that the increased friction coupled with the low weight of the projectile increases the effect of barrel length on the ballistic characteristics of the projectile. A longer barrel length can result in a slower muzzle velocity than a projectile fired through a shorter barrel potentially resulting in significantly different ballistic characteristics from barrel to barrel. Similarly, the metal rifling can strip away shavings from the softer polymer projectiles fouling the barrel. As a result, firearms used to fire lightweight polymer projectiles can require substantially more maintenance than firearms only firing conventional bullets and can create a potential safety risk if the fouling blocks barrel. Similarly, metal or composite pellets are typically used in air guns having rifled barrels as the plastic pellets will become caught within the barrel or be significantly damaged travelling through the barrel.
Known prior art practice ammunition, particularly 5.56 mm marking practice ammunition, designed for the AR-15 style rifles, have expanding telescoping cartridges and utilize a primer for propulsion or a primer in combination with a secondary propellent. The known commercial embodiments as tested provide kinetic energy levels above 62 either provide sufficient kinetic energy to cause damage to the environment, such as dings in walls, or have excessive weight and low velocity such that range and accuracy are diminished. No known prior art 5.56
Lightweight projectiles have significant advantages when used for non-lethal or training purposes. However, the inherent tradeoff between improved ballistic characteristics from increased muzzle velocities and increased standoff distance limit the usefulness of the projectiles. Similarly, the improved ballistic characteristics provided by the rifling of the barrel must be weighed against the inconsistent performance from barrel to barrel and the potential risk of fouling.